1. Field of the Invention
The present invention relates to a battery using, for example, lithium (Li) as a battery reacting species, and an anode material used in the battery.
2. Description of the Related Art
In recent year, in accordance with downsizing of electronic devices, a development of secondary batteries having a high energy density has been in demand. A secondary battery which meets the demand is a lithium secondary battery. However, in the lithium secondary battery, lithium is deposited on an anode to form a lithium dendrite during charge, thereby the lithium is inactivated, so there is a problem that the cycle life of the lithium secondary battery is short. Therefore, lithium-ion secondary batteries with improved cycle characteristics have been commercially available.
The lithium-ion secondary battery uses a graphite material which uses a intercalation reaction of lithium between graphite layers, or a carbonaceous material which uses an application of insertion/extraction reactions of lithium in pores, thereby lithium is not deposited to form a dendrite, and as described above, the cycle life of the lithium-ion secondary battery is longer. Moreover, the graphite material and the carbonaceous material are stable in air, so the lithium-ion secondary battery has a big advantage in industrial production.
However, in the graphite material, an anode capacity has an upper limit stipulated by the composition C6Li of a first stage graphite intercalation compound. Moreover, in the carbonaceous material, it is industrially difficult to control a minute pore structure thereof, and when the number of pores is increased, specific gravity of the carbonaceous material declines, so the anode capacity per unit volume cannot be improved. Further, it is known that some low-temperature sintered carbonaceous materials exhibit an anode discharge capacity exceeding 1000 mAh/g. However, when the battery comprises a metal oxide or the like having a large capacity at a noble potential of 0.8 V or over against lithium metal as a cathode, a discharge voltage declines. Because of these problems, it is considered that it is difficult for existing carbonaceous materials to meet a demand for a longer operating time of future electronic devices or a higher energy density of power sources. Therefore, a development of an anode active material having superior capability to insert and extract lithium has advanced.
As such an anode active material capable of achieving a higher capacity, for example, a material which can form an alloy with lithium such as zinc (Zn), cadmium (Cd), lead (Pb), tin (Sn), bismuth (Bi), silicon (Si), indium (In), antimony (Sb) or germanium (Ge), or a Li—Al alloy has been widely researched. Moreover, an anode using a Si alloy as the anode active material has been invented (refer to U.S. Pat. No. 4,950,566).
However, these anode active materials are expanded or shrunk in accordance with charge and discharge, thereby resulting in the materials being pulverized, so the cycle characteristics of the battery declines.
Therefore, in order to improve the cycle characteristics, an anode active material in which an element not involved in expansion and shrinkage in accordance with insertion and extraction of lithium is substituted for a part has been proposed. For example, LiSivOw (0≦v, 0<w<2) (refer to Japanese Unexamined Patent Application Publication No. Hei 6-325765), LixSi1-yMyOz (M represents metal except for alkali metal or metalloid except for silicon; 0≦x, 0<y<1, 0<z<2)(refer to Japanese Unexamined Patent Application Publication No. Hei 7-230800), and a Li—Ag—Te alloy (refer to Japanese Unexamined Patent Application Publication No. Hei 7-288130) have been proposed. However, even if any of these anode active materials is used, a decline in the cycle characteristics resulting from expansion and shrinkage of the alloys is large, so the fact is that full advantage cannot be taken of the characteristic, that is, a higher capacity.
Moreover, an anode active material using a compound including an Group 4B element except for carbon and one or more kinds of nonmetal elements has been proposed (refer to Japanese Unexamined Patent Application Publication No. Hei 11-102705), however, also in the anode active material, a decline in the cycle characteristics is large. Therefore, an anode using a mixture of metal or an alloy capable of forming an alloy with lithium and a carbonaceous material such as carbon fiber, carbon black or the like so as to be able to achieve a higher capacity and superior cycle characteristics has been developed (refer to Japanese Unexamined Patent Application Publication No. 2001-196052).
However, when the metal or the alloy capable of forming an alloy with lithium and the carbonaceous material are simply mixed, the rate of presence of the metal or the alloy capable of forming an alloy with lithium in the anode is reduced by the amount of the carbonaceous material mixed, so the capacity declines. Further, sufficient cycle characteristics cannot be obtained.